Brillouin fiber lasers operating in single longitudinal mode have been found useful for various applications including microwave signal processing, noise suppression, spectral narrowing, and so on. It has been shown that both acoustic damping and cavity feedback are responsible for phase noise reduction of the pump laser, thereby yielding much narrower spectral linewidth in single-frequency Brillouin fiber lasers. The spectral linewidth of a free running single frequency fiber ring laser can be only a few hertz (Hz), which is several orders of magnitude narrower than the pump beam.
So far, Brillouin fiber lasers have been constructed in the form of Fabry-Perot or ring resonators. However, such designs require hundreds of meters long fiber to obtain gain enough to overcome the loss and to reach the threshold. Because of such long lengths, existing Brillouin lasers tend to oscillate with multiple longitudinal modes, which degrade the performance of the laser for above mentioned applications by introducing additional noise.
Short cavity (about 20 meters (m)) Brillouin lasers that are doubly resonant to both pump and Stokes have been also been realized for single-mode operation with low pump power. However, they require active feedback control, which makes them difficult to operate. Another drawback of a conventional Brillouin fiber laser is that the first-order Stokes wave is subsequently converted to higher order Stokes waves, which affects the overall efficiency or performance.